Raman spectroscopy of cobalt ferrite nanocomposite in silica matrix prepared by sol–gel method
Introduction
Nanocomposite preparation techniques experienced a rapid development in the last years, for such materials support a wide variety of applications as compared to those prepared via conventional methods. The final properties of a nanoparticle-based composite are strongly dependent upon the final material morphology, which depends on the nanoparticle size, size dispersion and distribution of the nanosized phase throughout the hosting matrix. Nanocrystalline particles are unique once the high surface-to-volume ratio leads to physical and chemical properties different from their polycrystalline counterparts. In addition, nanocomposite materials prepared by sol–gel chemistry present high surface area due to its enhanced porosity, being attractive for applications such as insulators, ceramic precursors, and catalyst supports [1], [2], [3], [4]. The magnetic behavior and the optical properties of magnetic nanocomposites are strongly affected by the characteristics of both the embedded magnetic nanophase and the template. Nanocomposites containing maghemite (γ-Fe2O3) nanoparticles dispersed in silica templates have been synthesized by mechanical activation [5] or by heating a mixture of iron nitrate and silicon alkoxide between 700 and 900 °C [6]. Following these routes, however, it has been difficult to obtain a single magnetic phase, narrow-sized nanoparticle distribution, and uniform dispersion in the hosting template. In contrast, the alternative nanocomposite preparation route starting with previously synthesized nanoparticles dispersed in aqueous medium and latter on added to the template in a more controllable way is still barely explored [2], [7].
In this study, we describe the preparation and the optical characterization of CoFe2O4–SiO2 nanocomposites. The sol–gel nanocomposite synthesis used the aqueous-based magnetic fluid (MF) to introduce the CoFe2O4 nanoparticles into the hosting template whereas micro-Raman spectroscopy was used to investigate the interaction between the dispersed nanosized magnetic phase and the silica template. Raman measurements and porosity analysis were performed as a function of both the nanoparticle concentration and the annealing temperature, the later performed in the range of 300–1200 K.
Section snippets
Experimental
The nanocomposite samples were prepared using a two-step procedure. In the first step the previously synthesized aqueous CoFe2O4-based MF sample was dispersed in the tetraethoxysilane/ethanol/H2O/H+ sol–gel system (1:4:12:0.005 molar ratio). Stable aqueous CoFe2O4-based MF samples were obtained by chemical co-precipitation of Co(II) and Fe(III) ions in alkaline medium, following peptization in 0.5 mol/L perchloric acid, as described in the literature [8]. The sol–gel mixture precursory contained
Results
The non-polarized Raman spectra of PS300 and PS1200 samples (reference samples) are shown in Fig. 1. The PS1200 sample presented the typical Raman spectrum of vitreous SiO2[9]. Raman features peaking at 440, 805, 1065 and 1200 cm−1 are related to the silica matrix network. The two bands observed at 495 and 606 cm−1, named D1 and D2, are related to defects. The vibrations of these two bands involve pure oxygen motion [9]. The bands observed in the 900–1000 cm−1 range are assigned to OH impurity
Discussion
As far as the Raman features are concerned the spectra displayed in Fig. 1 show that the PS300 sample is quite different from the PS1200 sample. The intensities of the Raman bands related to silica network matrix of the sample dried at room temperature (PS300) are much weaker than the intensities of the Raman bands revealed by the sample annealed at a higher temperature (PS1200). It is also observed that as the annealing temperature increases the intensity of Si–OH related bands decreases,
Conclusion
In this study the synthesis and the optical characterization of thermal-treated nanocomposites based on CoFe2O4 nanoparticles dispersed in SiO2-based matrix is reported. Raman spectroscopy was used to investigate the interaction between CoFe2O4 nanoparticles and the hosting silica matrix in nanocomposites prepared by the sol–gel technique. Raman data recorded on samples annealed at room temperature indicate that Si–O–Fe (Co) bonds are formed when high concentration of the CoFe2O4 nanoparticles
Acknowledgements
The authors acknowledge the financial support of the Brazilian agencies FINEP/CTPETRO, CAPES, FUNAPE, FINATEC, and CNPq.
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